Cool stuff and UX resources

Why "how many users" is just the wrong question

May, 2007 Author: Kath Straub, PhD, CUA

Death. Taxes. How-many-users.

Every day in offices around the world usability professionals ask and are asked this question: How many users do we need for our usability test? Its an important question. We want to find most of and the most severe problems. So, we need to test enough people. But usability testing is so expensive, and the cost of testing increases with each participant. So, we don't want to test too many, either.

On the one hand, synthesizing the received theoretical wisdom suggests that there is an answer to this question. And answer is "5." (Virzi 1992; Nielsen and Landauer, 1992) That is, based on a probabilistic formula, you will need to test 5 users to find about 85% of the problems that will trip up 1/3 or more of your users. The number 5 is very concrete. Practitioners like it. 5 is easy to remember.

On the other hand, this question gets debated every year at the CHI conference. You can count on it.. Like death and taxes. The same debate. Given that the UX community (re-)debates this every year, it seems that the wisdom has not been so well received.

Blue! No, Green!... No, 5!

That the number 5 has such staying power says something interesting about human memory and the way people reason. The 5-formula can work. But, like tossing a coin, it's probabilistic. If you keep flipping a coin over and over, it will come up heads half the time. But it can also come up tails nine times in a row.

Similarly, if you run enough usability tests with 5 users, on average you will find most of the errors about most of the time. But if you run only one test (or just a few) with 5 users, it's possible that you will uncover fewer errors than the formula projects. (Spool and Schroeder, 2001; Faulkner, 2003, or you are less ambitious, there is the May, 2004 newsletter.)

There are other challenges with the 5-formula. For instance, to calculate the number of testing participants you need, a priori you need to know how many problems there are to find. If you knew that, likely you wouldn't need to test to find them, eh?

Reach beyond... # of users

Not surprisingly, the debate churned on in San Jose (CHI 2007). But this year, Lindgaard and Chattratichart (2007) threw down a different gauntlet. The obstacle to solving the problem, they said, is the question. "How many users" is the wrong way to think about it.

In usability testing, we are looking for mismatches between the site/app model and the user's mental model on the key and critical tasks. Framed this way, the criterion that determines how many problems get uncovered is how many tasks participants try, not how many participants there are.

To test their claim, Lindgaard and Chattratichart reanalyzed the usability testing data from CUE-4* (Molich, 2003 ‚Äď Workshop Reference). Within that project, 9 highly experienced teams used think-aloud techniques to independently test the same site. The teams received identical input from the coordinators (site objectives, problem criteria, testing focus). Each team shaped their own testing plan and protocol, conducted the testing, and aggregated the findings into a pre-determined feedback format.

Lindgaard and Chattratichart looked for similarities and differences across the methods and findings reported by each team. Specifically, they were seeking relationships between test design (e.g., # users, # tasks) and number of problems identified.

Their study reports that there was no reliable correlation between the number of users tested and the number of usability problems uncovered. Testing more users did not ensure that that more problems would be discovered. Further, although each of the 9 teams tested 5 users or more, they reported only 7-43% of the known problems, not the 85% predicted by the 5-formula.

In contrast, their analysis showed a significant positive correlation between the number of tasks evaluated and the number of problems uncovered. That is, the more tasks a team included in their testing protocol, the more problems they uncovered.

They conclude that other things being equal (e.g., quality of recruiting), the better predictor of the productivity of usability testing is the number of tasks participants (try to) complete, not the number of participants who try to complete them.

* The CUE Studies, Molich and Dumas, in press; Molich, Kaasgaard and Karyukin, 2004, among others, compare methods and findings of different teams conducting the same usability test. CUE findings show that different usability testing teams evaluating the same interface report different numbers usability problems, often with very little overlap in the identified. There's clearly more to it than number of users.

Message from the CEO, Dr. Eric Schaffer ‚ÄĒ The Pragmatic Ergonomist

This result is fantastic! It's like trying to find potholes in a city. Not every car hits every pothole in the road. So you need to send a number of cars down each road. But it is even more important to send cars down a larger NUMBER of roads. The key seems to be in more tasks, not just more users. The problem is that you can only run a given number of tasks with a single test participant. More than 60 or perhaps 90 minutes of testing won't work well.

I propose a "Lingaard-Chattratichart Testing Strategy." Test 3 different groups of participants. Put maybe 6 to 12 people in each group. Then have each group do a different basket of tasks. This will allow us to test a LOT of different tasks and should get a far better level of reliability.

Leave a comment here

Reader comments

Vikram Chauhan
UTV New Media Ltd

I have for years read the debates over which number of users is statistically significant, and yes the minimum of 5 has always been a safe bet. But seriously I don't see how increasing the number of tasks is more beneficial. I am a designer for applications for which I generally test a scope of tasks per feature and there is no need for more tasks when my goal is to a test specific set of features that can be completed with a finite set of tasks. Also the article references sites; in my experience the types of tests we perform to validate the usability of a site are often different than the types of measures used to test the usability of product applications. I think this distinction needs to be made.

Raju

Can you please comment on the selling of this idea to clients ‚Äď three groups of 6-12 participants. This would be helpful because every different user groups you recruit adds to the cost. Is it advisable to separate out as common tasks across groups and special tasks per specific group? Somewhere it also connects to the maturity of usability practice within an organization. Your recommendations can help practitioners in companies sell this to their own management or clients. If an end-to-end software solution provider needs the bandwidth to address usability in projects, are there more automatized tests or techniques that can be provided?

Subscribe

Sign up to get our Newsletter delivered straight to your inbox

Follow us

Privacy policy

Reviewed: 18 Mar 2014

This Privacy Policy governs the manner in which Human Factors International, Inc., an Iowa corporation (‚ÄúHFI‚ÄĚ) collects, uses, maintains and discloses information collected from users (each, a ‚ÄúUser‚ÄĚ) of its humanfactors.com website and any derivative or affiliated websites on which this Privacy Policy is posted (collectively, the ‚ÄúWebsite‚ÄĚ). HFI reserves the right, at its discretion, to change, modify, add or remove portions of this Privacy Policy at any time by posting such changes to this page. You understand that you have the affirmative obligation to check this Privacy Policy periodically for changes, and you hereby agree to periodically review this Privacy Policy for such changes. The continued use of the Website following the posting of changes to this Privacy Policy constitutes an acceptance of those changes.

Cookies

HFI may use ‚Äúcookies‚ÄĚ or ‚Äúweb beacons‚ÄĚ to track how Users use the Website. A cookie is a piece of software that a web server can store on Users‚Äô PCs and use to identify Users should they visit the Website again. Users may adjust their web browser software if they do not wish to accept cookies. To withdraw your consent after accepting a cookie, delete the cookie from your computer.

Privacy

HFI believes that every User should know how it utilizes the information collected from Users. The Website is not directed at children under 13 years of age, and HFI does not knowingly collect personally identifiable information from children under 13 years of age online. Please note that the Website may contain links to other websites. These linked sites may not be operated or controlled by HFI. HFI is not responsible for the privacy practices of these or any other websites, and you access these websites entirely at your own risk. HFI recommends that you review the privacy practices of any other websites that you choose to visit.

HFI is based, and this website is hosted, in the United States of America. If User is from the European Union or other regions of the world with laws governing data collection and use that may differ from U.S. law and User is registering an account on the Website, visiting the Website, purchasing products or services from HFI or the Website, or otherwise using the Website, please note that any personally identifiable information that User provides to HFI will be transferred to the United States. Any such personally identifiable information provided will be processed and stored in the United States by HFI or a service provider acting on its behalf. By providing your personally identifiable information, User hereby specifically and expressly consents to such transfer and processing and the uses and disclosures set forth herein.

In the course of its business, HFI may perform expert reviews, usability testing, and other consulting work where personal privacy is a concern. HFI believes in the importance of protecting personal information, and may use measures to provide this protection, including, but not limited to, using consent forms for participants or ‚Äúdummy‚ÄĚ test data.

The Information HFI Collects

Users browsing the Website without registering an account or affirmatively providing personally identifiable information to HFI do so anonymously. Otherwise, HFI may collect personally identifiable information from Users in a variety of ways. Personally identifiable information may include, without limitation, (i)contact data (such as a User‚Äôs name, mailing and e-mail addresses, and phone number); (ii)demographic data (such as a User‚Äôs zip code, age and income); (iii) financial information collected to process purchases made from HFI via the Website or otherwise (such as credit card, debit card or other payment information); (iv) other information requested during the account registration process; and (v) other information requested by our service vendors in order to provide their services. If a User communicates with HFI by e-mail or otherwise, posts messages to any forums, completes online forms, surveys or entries or otherwise interacts with or uses the features on the Website, any information provided in such communications may be collected by HFI. HFI may also collect information about how Users use the Website, for example, by tracking the number of unique views received by the pages of the Website, or the domains and IP addresses from which Users originate. While not all of the information that HFI collects from Users is personally identifiable, it may be associated with personally identifiable information that Users provide HFI through the Website or otherwise. HFI may provide ways that the User can opt out of receiving certain information from HFI. If the User opts out of certain services, User information may still be collected for those services to which the User elects to subscribe. For those elected services, this Privacy Policy will apply.

How HFI Uses Information

HFI may use personally identifiable information collected through the Website for the specific purposes for which the information was collected, to process purchases and sales of products or services offered via the Website if any, to contact Users regarding products and services offered by HFI, its parent, subsidiary and other related companies in order to otherwise to enhance Users‚Äô experience with HFI. HFI may also use information collected through the Website for research regarding the effectiveness of the Website and the business planning, marketing, advertising and sales efforts of HFI. HFI does not sell any User information under any circumstances.

Disclosure of Information

HFI may disclose personally identifiable information collected from Users to its parent, subsidiary and other related companies to use the information for the purposes outlined above, as necessary to provide the services offered by HFI and to provide the Website itself, and for the specific purposes for which the information was collected. HFI may disclose personally identifiable information at the request of law enforcement or governmental agencies or in response to subpoenas, court orders or other legal process, to establish, protect or exercise HFI‚Äôs legal or other rights or to defend against a legal claim or as otherwise required or allowed by law. HFI may disclose personally identifiable information in order to protect the rights, property or safety of a User or any other person. HFI may disclose personally identifiable information to investigate or prevent a violation by User of any contractual or other relationship with HFI or the perpetration of any illegal or harmful activity. HFI may also disclose aggregate, anonymous data based on information collected from Users to investors and potential partners. Finally, HFI may disclose or transfer personally identifiable information collected from Users in connection with or in contemplation of a sale of its assets or business or a merger, consolidation or other reorganization of its business.

Personal Information as Provided by User

If a User includes such User‚Äôs personally identifiable information as part of the User posting to the Website, such information may be made available to any parties using the Website. HFI does not edit or otherwise remove such information from User information before it is posted on the Website. If a User does not wish to have such User‚Äôs personally identifiable information made available in this manner, such User must remove any such information before posting. HFI is not liable for any damages caused or incurred due to personally identifiable information made available in the foregoing manners. For example, a User posts on an HFI-administered forum would be considered Personal Information as provided by User and subject to the terms of this section.

Security of Information

Information about Users that is maintained on HFI‚Äôs systems or those of its service providers is protected using industry standard security measures. However, no security measures are perfect or impenetrable, and HFI cannot guarantee that the information submitted to, maintained on or transmitted from its systems will be completely secure. HFI is not responsible for the circumvention of any privacy settings or security measures relating to the Website by any Users or third parties.

Correcting, Updating, Accessing or Removing Personal Information

If a User‚Äôs personally identifiable information changes, or if a User no longer desires to receive non-account specific information from HFI, HFI will endeavor to provide a way to correct, update and/or remove that User‚Äôs previously-provided personal data. This can be done by emailing a request to HFI at hfi@humanfactors.com. Additionally, you may request access to the personally identifiable information as collected by HFI by sending a request to HFI as set forth above. Please note that in certain circumstances, HFI may not be able to completely remove a User‚Äôs information from its systems. For example, HFI may retain a User‚Äôs personal information for legitimate business purposes, if it may be necessary to prevent fraud or future abuse, for account recovery purposes, if required by law or as retained in HFI‚Äôs data backup systems or cached or archived pages. All retained personally identifiable information will continue to be subject to the terms of the Privacy Policy to which the User has previously agreed.

Contacting HFI

If you have any questions or comments about this Privacy Policy, you may contact HFI via any of the following methods:
Human Factors International, Inc.
PO Box 2020
1680 highway 1, STE 3600
Fairfield IA 52556 hfi@humanfactors.com
(800) 242-4480

Terms and Conditions for Public Training Courses

Reviewed: 18 Mar 2014

Cancellation of Course by HFI

HFI reserves the right to cancel any course up to 14 (fourteen) days prior to the first day of the course. Registrants will be promptly notified and will receive a full refund or be transferred to the equivalent class of their choice within a 12-month period. HFI is not responsible for travel expenses or any costs that may be incurred as a result of cancellations.

Cancellation of Course by Participants (All regions except India)

$100 processing fee if cancelling within two weeks of course start date.

Cancellation / Transfer by Participants (India)

4 Pack + Exam registration: Rs. 10,000 per participant processing fee (to be paid by the participant) if cancelling or transferring the course (4 Pack-CUA/CXA) registration before three weeks from the course start date. No refund or carry forward of the course fees if cancelling or transferring the course registration within three weeks before the course start date.

Cancellation / Transfer by Participants (Online Courses)

$100 processing fee if cancelling within two weeks of course start date. No cancellations or refunds less than two weeks prior to the first course start date.

Individual Modules: Rs. 3,000 per participant ‚Äėper module‚Äô processing fee (to be paid by the participant) if cancelling or transferring the course (any Individual HFI course) registration before three weeks from the course start date. No refund or carry forward of the course fees if cancelling or transferring the course registration within three weeks before the course start date.

Exam: Rs. 3,000 per participant processing fee (to be paid by the participant) if cancelling or transferring the pre agreed CUA/CXA exam date before three weeks from the examination date. No refund or carry forward of the exam fees if requesting/cancelling or transferring the CUA/CXA exam within three weeks before the examination date.

No Recording Permitted

There will be no audio or video recording allowed in class. Students who have any disability that might affect their performance in this class are encouraged to speak with the instructor at the beginning of the class.

Course Materials Copyright

The course and training materials and all other handouts provided by HFI during the course are published, copyrighted works proprietary and owned exclusively by HFI. The course participant does not acquire title nor ownership rights in any of these materials. Further the course participant agrees not to reproduce, modify, and/or convert to electronic format (i.e., softcopy) any of the materials received from or provided by HFI. The materials provided in the class are for the sole use of the class participant. HFI does not provide the materials in electronic format to the participants in public or onsite courses.